1. Field of the Invention
The present invention relates to an optical head and an optical recording apparatus provided with an optical head, particularly to an optical head including a semiconductor laser and a light detecting device located within a single housing, and having the capability of emitting a light beam with high enough light power required to record data on an optical recording medium.
2. Description of the Related Art
An integrated optical head including a semiconductor laser, a light detecting device, and a hologram element that separates a light beam accommodated in a single housing has been recently proposed. This has practical applications in compact disk (CD) reproduction systems. FIG. 17 illustrates an example of such an optical head. The optical head includes a semiconductor laser 10 and a light detecting device 70 disposed adjacent to each other, as shown in FIG. 17. A forward light beam 801f emitted from the semiconductor laser 10 passes through a hologram element 30 as a zero order light beam (i.e., non-diffracted beam). The beam is focused by an objective lens 50 into a spot 61 on the recording surface of an optical recording medium 60. The light beam is reflected from the optical recording medium 60 and travels as a backward light beam 801r through the objective lens 50. The backward light beam 801r is diffracted by the hologram element 30 and focused onto a light detecting device 70.
The arrangement depicted in FIG. 17 has a finite optical system with a focusing element including only an objective lens 50. In an infinite optical system, a collimator lens is disposed between the hologram element 30 and the objective lens 50 so that the light beam becomes a parallel beam between the collimator lens and the objective lens. In any case, the light beam travels through the same optical system in both forward and backward directions and thus the magnification (or reduction ratio) associated with the forward light beam is the same as the magnification associated with the backward light beam. Thus, the numerical aperture (NA) for the forward light beam 801f is the same as that for the backward light beam 801r. Magnification is defined as the absolute value of lateral magnification. The reference of the lateral magnification is the side of a light source for a forward optical path, and it is the side of a light detector for a rearward optical path.
In general, the magnification of the optical system should satisfy requirements regarding the detection sensitivity of a focusing error signal. To read information pits having a very small size formed on an optical recording medium 60, a beam spot 61 having a diameter of about 1 .mu.m is necessary to accurately track the information pit on the recording surface of the optical recording medium 60. To satisfy this requirement, the focusing error should be less than .+-.1 .mu.m taking into account a small focal depth.
The light detecting device 70 detects the focusing error and produces a focusing error signal. The focusing error is corrected by controlling the position of the objective lens 50 with a focusing servo control using the focusing error signal from the light detecting device. This, however, requires a high resolution (detection sensitivity) of the focusing error signal. When the light detecting device 70 and the semiconductor laser 10 are assembled, there is some inevitable positioning error between the optical axis of these elements. This positioning error between the optical axes results in an initial offset of the focusing error signal. The initial offset d can be written as EQU d=D.times.(.beta..times..beta.).div.2
where .beta. is the magnification of the optical system, and D is the above-described positioning error between the optical axes. When the positioning error along the optical axis D is assumed to be .+-.50 .mu.m, then a magnification .beta. of 1/5 is necessary to have a small initial offset within an allowable range of the focusing error determined from the focal depth. In usual reproduction optical heads for use in CDs (compact disk systems), the magnification of the optical system is set to a value in the range from 1/4 to 1/6 for the reason described above.
As described above, the light beam must be focused via the objective lens 50 into a spot 61 with a size as small as 1 .mu.m. To meet this requirement, the NA (numerical aperture) of the objective lens 50 should be greater than 0.45 at its output side, provided that the semiconductor laser 10 emits a light beam having a wavelength .lambda. of 780 nm. However, if the NA.sub.0 is set to a greater value, that is, if the brightness of the image obtained by the lens becomes higher, then the focal depth becomes smaller and thus higher accuracy is required in the focusing servo control system beyond a degree that can be achieved practically. Thus, if the NA.sub.0 at the output side of the lens is set to 0.45, and the magnification of the optical system .beta. is set to 1/5, then the NA.sub.I at the input side becomes 0.45.times.1/5=0.09.
The semiconductor laser 10 emits a light beam having a far-field pattern in the form of Gaussian distribution in which the full width at half maximum in the direction parallel to the laser junction interface is about 10.degree., and the full width at half maximum in the direction perpendicular to that is about 25.degree.. Only part of the light beam having such a distribution can actually reach and enter the inside of the aperture 51 of the objective lens 50. In FIG. 17, as small as 20% of the light beam enters the aperture 51 when the NA.sub.I at the input side of the objective lens 50 is 0.09 and the transmission loss of the hologram element 30 is assumed to be negligible.
On the other hand, the total light power of the plurality of backward light beams 801r detected by the light detecting device 70 should be at least 20 .mu.W to achieve an acceptable signal-to-noise ratio associated with the detected signal. In the case of optical heads for use in reproduction of CDs (compact disks), a semiconductor laser 10 having a rather low capability in the rated output light power such as 5 mW is used to reduce cost.
An example of the design of an optical system regarding the distribution of light power is as follows:
Optical System Associated with the Backward Optical Path: ##EQU1## Optical System Associated with the Forward Optical Path: ##EQU2## where .alpha.0 is the zero order light efficiency of the hologram element, .alpha.1 is the first order light efficiency of the hologram element, and R is the reflectance of the optical recording medium. In the above example, it is assumed that the transmission loss of the optical system (due to for example the reflection at the surface of optical elements such as an objective lens) is 10%. Usually, the optical output light power available via the objective lens is about 0.3 mW. Since the hologram element also produces diffracted light of the second or higher order, the theoretical upper limit of the total efficiency including .alpha.0 and .alpha.1 is about 80%.
From the above equation, it can be seen that it is possible to design an optical system such that sufficient output light power can be obtained via an objective lens and thus a light detecting device can receive sufficient light power as long as the optical system is used in a read-only optical head. However, although it is possible to achieve satisfactory distribution of the light power for the read-only optical head, it is impossible to obtain consistent distribution of the light power of optical heads used to write information on an optical recording medium. The above difficulty occurs for the following reasons.
Various types of optical recording media having the capability of writing information are used in practical applications. These optical recording media are based for example on the magneto-optical technique, phase-change technique, or polymer-die technique. The write power, or the output light power available via an objective lens, required to write information on these media depends on the type of a recording medium used. In the case of a magneto-optical recording medium, 5 mW or greater write power is required. Unlike the read-only media, the reflectance of the writable optical recording medium decreases owing to the absorption of light by a recording layer. For example, the reflectance of a magneto-optical recording medium is as low as 16%. The read power, or the light power used in reading operations, should be as small as about 1 mW or a few tenths of the write power so as to prevent degradation in recorded information when reproduced repeatedly. If the above-described requirement is taken into account, an example of possible design regarding the light power distribution will be as follows.
Optical System Associated with the Backward Optical Path: ##EQU3## Optical System Associated with the Forward Optical Path: ##EQU4## In the above design example, it is assumed that the light detecting device receives the same light power as in the case of the read-only optical head described above. Furthermore, it is also assumed that the optical system has the same magnification .beta. and the same coupling efficiency .sup.n as in the case of the read-only optical head. From the above discussion, it can be concluded that the semiconductor laser used should have the capability of outputting very high light power such as 55 mW or greater. However, semiconductor lasers having such a high capability are not commercially available. Even if such a semiconductor laser becomes available in the future, it will be very expensive.
An attempt to solve the above problem is to employ an arrangement shown in FIG. 18, which is referred to as a second conventional optical head. An optical head is used in a writable optical recording apparatus instead of using an integrated-type optical head such as that described in FIG. 17. In this arrangement, a beam splitter 90 is used to separate the light beam separation means so that a semiconductor laser 91 and a light detecting device 97 can be located apart from each other. Thus, the optical system associated with the forward optical path and the optical system associated with the backward optical path are formed separately. In this arrangement, the magnifications associated with the forward and backward optical systems can be set independently of each other. In this arrangement, the magnification of the optical system associated with the forward optical path is given by the ratio of the focal length of a collimator lens 92 to that of an objective lens 95. This can be determined independently of the magnification of the backward optical system. When the magnification of the forward optical system is set to about 1/3, the NA associated with the forward light beam is 0.45/3=0.15, and thus the coupling efficiency .sup.n increases to about 45%. Thus ##EQU5## This power level permits the use of a commercially available semiconductor laser having the capability of outputting light power of 30 mW.
However, in the second conventional optical head described above, expensive optical elements such as a beam splitter are required. Furthermore, the forward and backward optical systems need separate installation spaces. As a result, it is very difficult to reduce the size of the optical head. This makes it difficult to reduce the size and cost of an optical recording apparatus and also makes it difficult to increase its operation speed.